1. Field of the Invention
The present invention relates to an H-mode drift tube linac which, by a TE-mode which excites a magnetic field in a charged particle transporting direction in an accelerator cavity, indirectly generates accelerating electric fields between a plurality of drift tube electrodes arrayed along a charged particle transporting direction, and accelerates charged particles, and to a method of adjusting an electric field distribution in the H-mode drift tube linac.
2. Description of the Background Art
An H-mode drift tube linac has two or more drift tube electrodes arrayed along the charged particle transporting direction (Z-axis direction) in an accelerator cavity which functions as a resonator to excite an H-mode, a gap being provided between each pair of the drift tube electrodes. The H-mode drift tube linac accelerates charged particles by indirectly generating an accelerating electric field in the gap between each pair of the drift tube electrodes.
The drift tube electrodes are hollow and have cylindrical shapes. Owing to an electric field generated at cylinder thickness parts of each pair (referred to as a cell) of the drift tube electrodes, accelerating energy is applied to charged particles, and then the accelerated particles pass through the inside of the drift tube electrodes. In this case, since in the accelerator cavity, a magnetic field is generated concentrically around the central axis of the accelerator cavity, an electric field distribution generated in the accelerator cavity owing to the magnetic field is, because of the H-mode, a sinusoidal distribution in which the intensity is minimum at the both ends of the accelerator cavity and is maximum at the middle thereof as viewed along the charged particle transporting direction (Z-axis direction).
The above electric field distribution in the accelerator cavity is in a state where the drift tube electrodes are not provided in the accelerator cavity. When the drift tube electrodes are provided in the accelerator cavity, since charged particles are yet to be accelerated and the velocities thereof are slower on the injection end side of the accelerator cavity than on the extraction end side thereof, the H-mode drift tube linac is designed such that the lengths of the drift tube electrodes are short on the injection end side. Therefore, since there are a relatively large number of the drift tube electrodes on the injection end side in the accelerator cavity, the electrostatic capacitance increases on the injection end side and the electric field distribution is such that the intensity is maximum at the injection end.
Such a concentration of the electric field distribution at the injection end side of the accelerator cavity causes, for example, a discharge between the drift tube electrodes, or heat generation in the accelerator cavity, resulting in hindering the linac from being stably used. Therefore, it is necessary to adjust the electric field distribution such that the maximum values of the electric field intensities at the gaps are uniform (flat) except at both the ends of the accelerator cavity, by, for example, optimally designing the inner diameter of the accelerator cavity, a tuner, or the like.
A radio-frequency phase at a time when charged particles arrive at the middles of the gaps is referred to as a synchronous phase, and charged particles are influenced so as to focus or defocus depending on a choice of the synchronous phase. Here, the radio-frequency phase has a period of 180 degrees which is from −90 degrees to +90 degrees, and the electric field intensities are generated so as to have a cosine waveform.
It is known that, in the charged particle transporting direction (Z-axis direction), according to a principle of phase stability, charged particles are focused by choosing a negative phase (from −90 degrees to 0). This is because, since a negative synchronous phase is a region in which the electric field intensity increases with time, particles which have arrived at a gap are subjected to a stronger electric field intensity than preceding particles which have passed the gap, and catch up with the preceding particle, whereby charged particles are focused. Contrariwise, when a positive phase (from 0 to +90 degrees) is chosen, charged particles are defocused in the charged particle transporting direction.
On the other hand, in the radial direction perpendicular to the Z-axis direction, charged particles are focused by choosing a positive phase (from 0 to +90 degrees) from the shape of lines of electric force generated between each pair of the drift tube electrodes. This is because, since the shape of the lines of the electric force is a curved shape in which the lines are centrally directed in the radial direction in the front half of the gap, and are directed outward in the radial direction in the back half of the gap, charged particles are subjected to a stronger electric field intensity in the front half of the gap than in the back half of the gap owing to a positive synchronous phase, whereby charged particles are focused in the radial direction. Contrariwise, when a negative phase (from −90 degrees to 0) is chosen, charged particles are defocused.
As described above, when a positive phase is chosen, charged particles are defocused in the charged particle transporting direction, and contrariwise, focused in the radial direction. When a negative phase is chosen, charged particles are focused in the charged particle transporting direction, and contrariwise, defocused in the radial direction. Therefore, by varying the positive and negative sign of the synchronous phase with a cycle of several cells, charged particles can be focused both in the charged particle transporting direction and in the radial direction.
One example of such a self-focusing method is an APF (Alternating Phase Focused) method. An H-mode drift tube linac adopting the APF method uses the accelerating electric field not only for acceleration but also for focusing. Therefore, the fabrication tolerance for the design value of the electric field distribution (that is, fabrication accuracy of the accelerator cavity) becomes strictly.
Therefore, in conventional art, there are proposed, for example, an electric field distribution adjusting method (e.g., see Japanese Laid-Open Patent Publication No. 2007-157400) using a tuner, an electric field distribution adjusting method (e.g., see Japanese Laid-Open Patent Publication No. 2006-351233) based on the shapes of the drift tube electrodes, or a method (e.g., see Japanese Laid-Open Patent Publication No. 2007-87855) of adjusting only a resonance frequency so as not to vary the electric field distribution which has been once set.
Thus, in order to adjust the electric field distribution such that the maximum values of the electric field intensities at the gaps are uniform (flat) except at the both ends of the accelerator cavity, as a premise, it is necessary to measure, in advance, the distribution of the electric fields generated between the respective pairs of the drift tube electrodes in the accelerator cavity. As a method for such electric field distribution measurement, a perturbation method is known. In the perturbation method, a small measurement sphere is inserted along the charged particle acceleration axis in the accelerator cavity. Then, disturbance of the electric fields, generated at this time, slightly fluctuates energy accumulated in the accelerator cavity, and a resonance frequency varies along with the fluctuation. From the variation amount of the resonance frequency, the electric field intensity at a place where the measurement sphere is positioned is calculated.
Upon application of the perturbation method, a perturbation sphere is fixed to one end of a string to insert the perturbation sphere into the accelerator cavity, the other end of the string is connected to a motor placed outside the accelerator cavity, the perturbation sphere fixed to the string is inserted into the accelerator cavity by the motor driving (e.g., see Alternating-phase-focused IH-DTL for an injector of heavy-ion medical accelerators, Y. Iwata, et al., Nuclear Instruments and Methods in Physics Research Section A: Volume 569, 2006, Pages 685-696).
When the electric field distribution in the accelerator cavity is measured by adopting the above perturbation method, since it is necessary to insert the perturbation sphere from the outside of the accelerator cavity, the inside of the accelerator cavity should be at the atmospheric pressure. Therefore, the electric field distribution generated when the linac is actually operated after the inside of the accelerator cavity is vacuumized and a radio-frequency power is fed, cannot be measured at all.
Thus, for example, when there arises a problem that charged particles satisfying a specification are not extracted because the electric field distribution varies during operation owing to an heating variation or a thermal variation of the structure of the accelerator cavity, the following need and trouble arise conventionally. That is, there arises a need to, after all apparatuses connected to the front or the back of the accelerator cavity are removed and vacuum is released, measure again the electric field distribution in the accelerator cavity by the perturbation method, and confirm whether or not the electric field distribution between the drift tube electrodes in the accelerator cavity is generated in accordance with the designing, and thereby a trouble such as extra labor of measurement and confirmation, arises.